The one metre lens of the Swedish Solar Telescope on La Palma. Photo: Dan Kiselman.

Vacuum telescopes cannot be built with very large apertures because such a design requires a vacuum window of extremely high optical quality, one that is as large as its aperture. It is then the first optical component in the telescope and has to be polished to high optical quality. This is difficult because of the thickness needed to withstand the enormous difference in pressure between the air side and the inside of the vacuum window.

The practical limit for vacuum telescopes is probably a little over 1 meter in diameter or slightly larger than the SST.

Temperature controlled optics

For future very large solar telescopes, the only viable technique seems to be to build open air telescopes with temperature controlled optics. Examples of telescopes that use or plan to use such design are the 1.5 m GREGOR, on Tenerife, the 1.6 m Goode Solar Telecope on Big Bear, and the 4 m Daniel K. Inouye Solar Telescope, now being built in Hawaii.

Gregor
Goode Solar Telescope
Daniel K Inouye Solar Telescope

Vacuum telescope

The SST is a vacuum telescope. Instead of a flat vacuum window, it uses a 1-metre diameter lens to seal off the vacuum. By using a lens of a single glass, excellent image quality is obtained through very narrow filters, that isolate a single wavelength or color.

This is important to solar physicists because the solar spectrum is rich with narrow spectral lines, some of them covering less than 1/2000 of the visible colors. However, looking through such a telescope without filters would produce a poor image, because different colors are focused at different distances from the lens.

Schupmann telescopes

For other observations that require broad wavelengths to be observed simultaneously, the SST can redirect light from a small part of the Sun to a corrector that puts all colors together at a single focus.

Telescopes that use such a corrector are called Schupmann telescopes after Ludwig Schupmann, who proposed such optical designs 100 years ago. This may sound very old-fashioned but in fact all modern large night-time telescopes are based on the Cassegrain or Gregory designs from the 17th century.

Ludwig Schupmann

Is the SST a Schupmann telescope?

Because the SST allows either direct use of the singlet lens to form an image or lets the light pass through a corrector of the Schupmann type, it cannot be described completely as a Schupmann telescope. Its design allows the astronomer to choose between two optical systems depending on the application.

Top of Schupmann corrector tube. The Swedish 1-m Solar Telescope on La Palma. Photo: Dan Kiselman.